This invention relates to transducers used to excite and receive acoustic waves in a metal workpiece, and, more particularly, to noncontact transducers that do not require a couplant between the transducer and the workpiece.
A variety of techniques are known for analyzing and characterizing materials and workpieces, and then predicting their subsequent service behavior. For many years, the techniques had been based upon destructive testing, wherein a piece of the material was evaluated by a methodology which then rendered it unfit for further service, such as mechanical testing to failure or metallography. From the destructive test results, predictions are made for the performance of similar parts in service. Of course, the actual service part cannot be evaluated directly, because it is destroyed in the testing procedure. The viability of this type of testing depends upon the ability to predict the performance of one part from tests made on another part, and materials scientists and engineers have long recognized that in many instances such predictive capability is not yet available.
Nondestructive testing, wherein a part or workpiece is evaluated by a procedure that does not damage or destroy it, has become of greater interest as materials are used in ever more demanding applications. The predictive inaccuracies associated with destructive testing become more significant when smaller design margins are available. Nondestructive testing offers a way to evaluate the very workpiece that is to be used in service, thereby avoiding many of the predictive problems in going from the tested piece to the service piece. A number of nondestructive test procedures have been developed, and some are now regularly used. One continuing effort has been to improve nondestructive testing procedures and to broaden the scope of operations that can benefit from the use of such techniques.
One of the most widely used nondestructive test techniques is ultrasonic testing, in which a sound wave, typically of a frequency that is too high to be heard by the human ear, is introduced into a workpiece and then received and evaluated after it has passed through a portion of the piece. Some properties of the workpiece, such as its modulus of elasticity, can be determined directly from the received sound wave, while others require more sophisticated analysis of the wave. Ultrasonic testing is now widely used to predict the remaining life expectancy of a part actually in service, and from these results develop better materials and production methods for extending the operating lives of subsequently produced parts.
A critical part of an ultrasonic testing system is the transducers that produce sound waves in the workpiece, and receive sound waves from the piece. These transducers must be able to function with waves of the desired frequency, under the required environmental conditions. Families of transducers have been developed for such testing. Typically, such transducers are piezoelectric transducers, using a crystal wherein an introduced electric signal produces a mechanical movement of the crystal. The crystal is acoustically contacted to the workpiece, either directly or indirectly with a coupling medium, so that the movement is transmitted into the workpiece, resulting in an acoustic wave. The received wave can similarly be detected with a piezoelectric crystal, with a small mechanical movement of the acoustic wave producing a measurable electric signal. Such transducers are widely and successfully used in many applications.
However, there are other applications where contacting transducers cannot be readily used. For example, if a part is used at high temperatures or pressures, in high levels of radiation, or in other adverse environments, then it is desirable to be able to actually test it at that temperature while in service. Contacting transducers often cannot be used under these conditions, because of the inability to couple the transducer to the workpiece, or because the environment damages the transducer.
There are now available noncontacting ultrasonic transducers. Such transducers permit introduction or reception of sound waves without being physically coupled to the workpiece (although the transducer may lightly touch or rest upon the workpiece without a coupling medium therebetween). The most important type of such transducer is the electromagnetic acoustic transducer, or EMAT. The EMAT includes a large magnet that applies a high intensity magnetic field to the surface of the metallic workpiece, and an electromagnetic excitation or eddy current coil operating above the surface in the magnetic field. It is not necessary that either coil actually touch the workpiece. When a signal is applied to the eddy current coil, a corresponding signal is transmitted into the surface of the workpiece. Similarly, a wave in the surface may be detected with an EMAT placed above the surface.
EMATs have the potential of extensive use in a variety of nondestructive testing applications, but current versions have some important drawbacks. Existing EMATs are bulky and difficult to package as a small, versatile unit. There have been proposed no effective EMAT designs for use in highly adverse environments such as high temperatures. The existing EMATs can produce a variety of types of ultrasonic waves in the workpieces. By contrast, the existing contact-type transducers are much smaller and more convenient to use, but can produce only a limited number of ultrasonic wave types. Finally, the power of existing small EMATs is limited by their design.
Accordingly, there exists a need for an improved electromagnetic acoustic transducer that is compact yet powerful, operable in adverse environments, and versatile. The present invention fulfills this need, and further provides related advantages.